This invention relates generally to a control system for a drive system that includes an engine and a continuously variable transmission, and more particularly, to a control system for a hydromechanical continuously variable transmission which controls displacement of a pump, displacement of a motor, and engagement of appropriate clutches to regulate engine load.
Many machines, particularly earth working machines, use a hydrostatic drive system to drive the traction wheels or tracks of the machine. Frequently, the prime mover engine of the machine is set to operate at a predetermined speed via an engine throttle. The machine speed can then be regulated by controlling the displacement of one of the hydraulic elements of the hydrostatic drive system, without adjusting the engine throttle.
One problem common to many known hydrostatic drive systems is that operation of the hydraulic elements produce loads on the engine that causes engine xe2x80x9cstallingxe2x80x9d or xe2x80x9cluggingxe2x80x9d, a.k.a., excessive engine speed droop thereby decreasing the productivity of the work machine. For most work machines, the operator is required to control the engine load manually by controlling the displacement of the hydraulic elements. However, this is tedious and often difficult. For other work machines, electronic controllers are used to control the displacement of the hydraulic elements in response to engine speed. Such systems may become unstable because the time required to make necessary adjustments to the hydraulic elements is much greater that the time necessary to generate a command signal in response to changes in engine speed.
The present invention is directed toward overcoming one or more of the problems as set forth above.
In accordance with a first aspect of the present invention, there is provided an apparatus for controlling load on an engine. The apparatus includes a continuously variable transmission driven by the engine. The apparatus further includes an engine speed sensor being adapted to sense the output speed of the engine and produce an engine speed signal indicative of the engine output speed and a travel speed sensor being adapted to sense the travel speed of the machine and produce a travel speed signal indicative of the machine travel speed. The apparatus yet further includes an electronic controller operable to compare the engine speed signal to a first underspeed value and produce an integral error signal indicative of the difference between the engine speed signal and the first underspeed value, supply an integral gain value based on the machine travel speed, modify the integral error signal through a proportional plus integral feedback controller to produce an underspeed request signal, and produce a command signal based on the underspeed request signal. The command signal is used to control the transmission ratio of the continuously variable transmission so as to control the load on the engine.
In accordance with a second aspect of the present invention, there is provided a method for controlling a load on an engine associated with a continuously variable transmission. The method includes the steps of sensing the rotational speed of the engine and producing an engine speed signal indicative of the rotational engine speed and sensing the travel speed of the machine and producing a travel speed signal indicative of the machine travel speed. The method further includes the steps of comparing the engine speed signal to a first underspeed value and producing an integral error signal indicative of the difference between the engine speed signal and the first underspeed value. The method yet further includes the steps of determining an integral gain value based on the machine travel speed signal and the integral error and modifying the integral error signal through a proportional plus integral feedback controller to produce an underspeed request signal. The method still further includes the steps of generating a command signal based on the underspeed request signal and controlling the transmission ratio of the continuously variable transmission with the command signal so as to regulate the load on the engine.